Electron-affinic Radiosensitizers Possessing NPSH-reactive side Chains: Cytotoxicity and Radiosensitizing Activity Towards Hypoxic EMT6 Cells in Vitro

Abstract

A new class of dual-function nitroazole derivatives that are composed of electron-affinic nitroazole rings and a thiol-reactive α,β-unsaturated carbonyl side chain were synthesized to evaluate their physico-chemical properties, reactivity with glutathione (GSH), and cytotoxicity and radiosensitizing activity towards EMT6/KU cells in vitro. Among this class of nitroazole compounds, 2-nitroimidazole derivative (1), 3-nitro-1,2,4-triazole derivative (2), and 2-methyl-4-nitroimidazole derivative (3) with a common side-chain structure of trans CH₂CH = CHCO₂CH₃ readily reacted with GSH in phosphate-buffer solution (pH 7·2, 310 K). These compounds showed higher cytotoxicities to both aerobic and hypoxic EMT6/KU cells than the corresponding α,β-saturated counterparts (6–8) with a side-chain structure of CH₂CH₂CH₂CO₂CH₂CH₃. The hypoxic cytotoxicity of 1 and 2 with similar electron affinities to that of misonidazole (9) was potentiated by the combined effects of depletion of non-protein thiols (NPSH) by the α,β-unsaturated carbonyl side chains and bioreduction of the nitroazole rings. The sensitizer enhancement ratios in vitro (SERvitro) of 1 (2·80 ± 0·20) and 2 (2·63 ± 0·27) at a dose of 1·0 mmol dm⁻³ are comparable with the oxygen enhancement ratio (OER = 2·90 ± 0·10) and are significantly larger than those of their respective counterparts 6 (1·28 ± 0·06) and 7 (1·22 ± 0·09). A less electron-affinic compound, 3, also gave a large SER_vitro = 1·80 ± 0·18, whereas the counterpart 8 was not effective (1·10) in radiosensitizing hypoxic cells. Compounds 1–3 not only altered the slope, but also reduced the shoulder of the dose–survival curve. These ‘dual-function’ nitroazole radiosensitizers show much lower levels of in vitro radiosensitization, as measured by the C_1·6, than the previously reported ‘anomalous’ radiosensitizers such as 5-substituted 4-nitroimidazole radiosensitizers.